The invention elates to self-locking threaded connections of the general character described in the commonly owned copending patent application Ser. No. 07/652,553. Typical examples of threaded connections to which the present invention pertains are those between a spindle, bolt or screw and a single nut, or between a spindle, bolt or screw and a first nut as well as a second or lock nut.
German Pat. No. 26 45 519 to Holmes discloses a self-locking threaded connection wherein the roots of the internal and external threads are provided with ramps which can be engaged by the crests of the external and internal threads, respectively, to establish a self-locking connection in response to axial stressing of the male and/or female component of the threaded connection. The ramps are inclined relative to the central axes of the respective threads at an angle of between 15.degree. and 45.degree., preferably 20.degree. to 35.degree.. A drawback of the patented connection is that a self-locking action can be achieved in one direction only. On the other hand, it is often desirable to establish a self-locking connection which is effective in both axial directions. Another drawback of the patented proposal is that unidirectional self-locking action can be achieved only in response to extensive axial stressing (and accompanying displacement) of at least one of the mating parts.
U.S. Pat. No. 4,586,861 to McKewan discloses a threaded connection wherein the ramp at the root of one of the threads has a convex shape. The purpose of the convex ramp is to enhance the resistance against vibrations. A drawback of this patented connection is that, once deformed, the crest of the thread which engages the ramp can slide in a direction toward the deepmost portion of the root of the other thread.
Published German patent application No. 29 03 845 of Holmes discloses a threaded connection wherein the inclination of the faces on the external thread departs from the inclination of the adjacent faces on the internal thread. In addition, the root of the internal thread is provided with a flat, and the maximum diameter of certain portions of the root of the internal thread is less than the maximum diameter of a portion of the crest of the external thread. The purpose of the threaded connection of Holmes is to reduce the likelihood of loosening in response to vibration.
The self-locking action of mating external and internal threads is attributable to elastic deformation of abutting faces of the two threads. The locking action remains intact as long as the abutting faces are urged against each other as a result of axial stressing of the male and/or female component.